WO2021043400A1

WO2021043400A1 - A system and a method for determining a feedback torque to be applied to a steering wheel

Info

Publication number: WO2021043400A1
Application number: PCT/EP2019/073617
Authority: WO
Inventors: Xabier CARRERA AKUTAIN
Original assignee: Toyota Motor Europe
Priority date: 2019-09-04
Filing date: 2019-09-04
Publication date: 2021-03-11

Abstract

A system and a method for determining a feedback torque to be applied to a steering wheel configured to be used in a steering system of a vehicle, comprising: acquiring measured values of an angle (AA) and of an angular rate (BB) of the steering wheel, and of a torque (CC) associated with the steering wheel, acquiring, using a virtual model of the steering system and of the vehicle, calculated values of an angle (DD) and of an angular rate (EE) of the steering wheel, and of at least one calculated torque (FF) value associated with the steering system determining (217) said feedback torque to be applied using the measured values and the calculated values.

Description

A system and a method for determining a feedback torque to be applied to a steering wheel

Field of the disclosure

The present disclosure is related to the field steering wheels and more precisely to the control of a feedback torque to be applied on a steering wheel, either in a simulation system or a vehicle.

Description of the Related Art

Simulators may be used to simulate the operation of a vehicle. In order to be realistic, these simulators may comprise a module which applies a feedback torque to the steering wheel which reflects the torque that may result from the operation of the vehicle on a surface (for example of a vehicle equipped with mechanical steering).

Also, vehicles equipped with steer by wire modules or vehicles equipped with power steering modules (for example EPS, Electric Power Steering modules) may also comprise a module which applies a feedback torque to the steering wheel which reflects the torque that may result from the operation of the vehicle on a surface (for example of a vehicle equipped with mechanical steering).

Determining the value of this feedback torque is particularly critical to obtain a realistic feel for users.

Known methods still apply unrealistic torques, for example with oscillations.

From the prior art, document US 2006/0086560 discloses a method in which a steering wheel model is used to calculate a desired steering wheel angle. The method calculates a motor command value for a steering angle servo-actuator. The model used in this document is not complete enough to be satisfying. Also, this document is aimed at altering driver feel which has been observed by the inventors of the present invention as not satisfying.

Also, from the prior art, document US 2003/0069676 is known and this document discloses an algorithm implemented using two control units wherein a switch between a first and a second control signal is determined by means of a changing unit. This changing unit selects a control signal according to whether or not it is considered that the behavior of the vehicle is abnormal. If the behavior of the vehicle is considered abnormal, then a torque is calculated on the basis of the steering wheel angle and then applied. This document is silent on how to determine when the vehicle behavior is abnormal and this solution is therefore not satisfactory.

Document US 2019/0100236 is also known and this document discloses an algorithm which detects traction steer events by comparing a deviation between the steering system data measured from the sensors of the vehicle to pre-determined (hard-coded) values stored in look-up tables. This document also discloses the triggering of a warning signal. This document fails to propose a solution which increases steering feedback fidelity.

Summary of the disclosure

The present disclosure overcomes one or more deficiencies of the prior art by proposing a method for determining a feedback torque to be applied to a steering wheel configured to be used in a steering system of a vehicle, comprising: acquiring measured values of an angle and of an angular rate of the steering wheel, and of a torque associated with the steering wheel, acquiring, using a virtual model of the steering system and of the vehicle, calculated values of an angle and of an angular rate of the steering wheel, and of at least one calculated torque associated with the steering system, determining said feedback torque to be applied using the measured values and the calculated values.

The inventors of the present invention have observed that using torque (measured and calculated using a virtual model) as input allows determining a feedback torque which is more realistic than a feedback torque calculated on the basis of, for example, a steering angle.

Determining said feedback torque may comprise comparing at least one measured value with the associated calculated value and/or comparing at least another value obtained using at least one measured value with the another value obtained using the associated calculated value.

For example, the method may be implemented using a calculator and actuators which can apply said feedback torque to be applied.

Also for example, the method may comprise a closed-loop regulation based on at least one of the measured value and the associated calculated value.

It should be noted that the measured torque may be a torque measured at any location on the steering column and the steering wheel or elements thereof. Also, the person skilled in the art will know how to implement a virtual model of the steering wheel system according to the application, this model being able to deliver at least the calculated values.

According to a particular embodiment, the system further comprises determining a first torque balance using at least the measured torque associated with the steering wheel and a compensation torque compensating a disturbance, the first torque balance delivering a value of the torque applied by the driver.

The person skilled in the art will know how to determine a torque balance. The torque applied by the driver is the torque at the driver's hands if the driver is holding the steering wheel. Determining this torque has been observed as being particularly efficient to obtain a realistic feedback torque. This torque applied by the driver may then be used in any closed- loop regulation.

According to a particular embodiment, the compensation torque compensating a disturbance is obtained by determining the difference between the calculated and the measured angle values and the difference between the calculated and the measured angular rate values. It has been observed that the compensation torque obtained in this manner is a good compensation to suppress unwanted oscillations in the torque.

According to a particular embodiment, determining said feedback torque to be applied to the steering wheel is performed using at least the compensation torque compensating a disturbance, and the at least one calculated value of the torque associated with the steering wheel.

According to a particular embodiment, the method further comprises determining a second torque balance using at least the value of the torque applied by the driver and/or the calculated value of the at least one torque associated with the steering wheel, so as to obtain at least a calculated angular acceleration value of the steering wheel.

It should be noted that there may be no first torque balance, for example if the method is applied to a power steering context. The second torque balance may then be renamed as first torque balance. According to a particular embodiment, the calculated angular rate and the calculated angle are respectively obtained by integrating the calculated angular acceleration and integrating the calculated angular rate.

According to a particular embodiment, determining the feedback torque to be applied to the steering wheel comprises determining a third torque balance using the values obtained from the calculated angular acceleration value of the steering wheel and/or the compensation torque.

The third torque balance determines the feedback torque to be applied. According to a particular embodiment, determining the feedback torque to be applied to the steering wheel comprises taking into account a friction parameter of the steering wheel and/or a damping parameter of the steering wheel.

For example, a previously calculated angular rate may be multiplied by a damping coefficient (the damping parameter) to obtain a torque representing the damping, and this torque may then be used in another torque balance.

Also, a torque resulting from friction may be determined on the basis of a calculated angular rate and of a calculated angle and this friction parameter.

These friction parameter and damping parameter may be used to obtain an even more realistic feedback torque.

According to a particular embodiment, the steering wheel is a steering wheel of a simulator. This simulator comprises at least one actuator acting on the steering system, and it may also comprise a screen.

According to a particular embodiment, the steering wheel is a steering wheel of a vehicle equipped with a steer by wire module.

According to a particular embodiment, the steering is a steering wheel of a vehicle equipped with a power steering system of a vehicle.

The invention also proposes a device for determining a feedback torque to be applied to a steering wheel configured to be used in a steering system of a vehicle, comprising: a module for acquiring measured values of an angle and of an angular rate of the steering wheel, and of a torque associated with the steering wheel, a module for acquiring, using a virtual model of the steering system and of the vehicle, calculated values of an angle and of an angular rate of the steering wheel, and of at least one calculated torque associated with the steering system, a module for determining said feedback torque to be applied using the measured values and the calculated values. This device may be configured to implement all the embodiments of the method as defined above.

The invention also proposes a system comprising the device as defined above.

This system may be a simulator or a vehicle. In one particular embodiment, the steps of the method are determined by computer program instructions.

Consequently, the invention is also directed to a computer program for executing the steps of a method as described above when this program is executed by a computer. This program can use any programming language and take the form of source code, object code or a code intermediate between source code and object code, such as a partially compiled form, or any other desirable form.

The invention is also directed to a computer-readable information medium containing instructions of a computer program as described above.

The information medium can be any entity or device capable of storing the program. For example, the medium can include storage means such as a ROM, for example a CD ROM or a microelectronic circuit ROM, or magnetic storage means, for example a diskette (floppy disk) or a hard disk.

Alternatively, the information medium can be an integrated circuit in which the program is incorporated, the circuit being adapted to execute the method in question or to be used in its execution.

Brief description of the drawings

How the present disclosure may be put into effect will now be described by way of example with reference to the appended drawings, in which:

- figure 1 is a representation of the different elements modeled by a virtual model of the steering system and of the vehicle,

- figure 2 is a representation of an example of implementation,

- figure 3 shows another example of implementation, - figure 4 is a graphical representation of the evolution of the torque according to the steering angle using a method according to an example and a method according to the prior art,

- figure 5 is a graphical representation of the power spectral density using a method according to an example and a method according to the prior art,

- figure 6 is a schematic representation of a system including a device according to an example.

Description of the embodiments

An exemplary method for determining a feedback torque to be applied to a steering wheel configured to be used in a steering system of a vehicle will now be described.

While generally known in themselves by the person skilled in the art, virtual models of steering system and of a vehicle represent elements such as the ones shown on figure 1. While the virtual model may implement a set of equations, its conception is based on the representation of the operation of a steering system of a vehicle and of at least some of its components. An example of these components is shown on figure 1, with the associated parameters that are delivered by the steering model. The example of figure 1 is non-exha ustive and the model can be used to simulate additional components.

On this figure, the virtual model 100 is able to simulate a rack 101, and this rack is associated with the following parameters: Assis_t , the assist force, for example if the vehicle is equipped with a power steering module 102, i_g, the gearing ratio between the motor rotation and the rack displacement,

Rack_Fl, the left rack force coming from the left wheel of the vehicle, Rackp_rr the right rack force coming from the right wheel of the vehicle, Xrac_k , the rack displacement,

Crac_k , a damping coefficient associated with the rack,

^Mrac_k, the mass of the rack,

Ffrac_k , the friction force associated with the rack, and i_p, the gearing ratio between the rack displacement and the steering column bar rotation (the column being defined as comprising all the elements between the steering rack and the steering wheel).

The virtual model is also able to simulate a steering column 103 coupled to the rack and associated with the following parameters: K_tb, the torsion bar stiffness,

K_coU the column stiffness (of only the portion between the torsion bar and the steering wheel),

5_co the column angle,

T_fco the friction force associated with the column, C_co the damping coefficient associated with the column, It should be noted that the model may deliver an upper column torque 7¾.

Finally, the virtual model is able to simulate a steering wheel 104 which is associated with the following parameters: T_sw, the steering wheel torque, l_sw, the inertia of the steering wheel,

5_swr the steering wheel angle,

5_swr the steering wheel angular rate, and the friction force associated with the steering wheel. All the above-defined parameters are merely given as examples and more parameters may be delivered by a model. The parameters which are calculated using the model may be used in a method for determining a feedback torque to be applied to a steering wheel.

In fact, the methods for determining a feedback torque according to the invention comprise a step of acquiring calculated values as defined above using a virtual model. These methods also comprise a step of acquiring measured values on the corresponding real world elements.

Figure 2 shows the steps 200 of a method according to an example. This method may be carried out for a simulator equipped with a steering wheel or for a vehicle using a steer by wire system.

A first torque balance is determined in step 201 using as input a measured torque associated with the steering wheel T™^eas. Any suitable torque sensor may be used to obtain this value.

The other input of the first torque balance is a torque determined using the measured angular rate of the steering wheel 5™f^as derived over time (step 202) and multiplied by a known steering wheel inertia

A third input of the first torque balance is a compensation torque compensating a disturbance at the steering wheel T_comp.

The first torque balance performs the following operation:

Wherein T^^iv is the value of the torque applied by the driver.

A second torque balance is determined in step 204 using at least as inputs T^^iv and T^_o1 the calculated torque of the upper column obtained using a model such as the one of figure 1. The second torque balance may use other inputs which will be described hereinafter. The second torque balance delivers a torque which will then be multiplied by l/(s/s_w ^rt) (step 205) wherein

is the virtual steering wheel inertia (the invention advantageously allows differentiating the real and the virtual steering wheel inertias). Then, a calculated angular rate of the steering wheel

is obtained through a first integration step.

Through a second integration step 206, a calculated angle of the steering wheel

is obtained.

Step 207 is the determination of the compensation torque T_comp. In this step, comparisons 208 and 209 are first carried out between

and 5 ^as and between

and 8 ^as . The outputs of these comparisons are multiplied by correctors Kp (step 210) and Kd (step 211), having a value determined for example through calibration.

A calculation is then carried in step 212:

The obtained compensation torque is used to compensate a disturbance (i.e. suppress) affecting the driver's feel. Thus, T_comp is used to minimize the mismatch between the dynamics of the virtual steering model and the physical dynamics of the control loading.

The above obtained calculated angular rate

may also be derived over time (step 213) and multiplied by an inertia value ] ^^f equal to the difference between the real and the virtual inertia of the steering wheel (step 214). This allows taking into account the possible differences between the real and the virtual steering wheels. Also, the angular rate can be multiplied by a damping coefficient C_sw of the steering wheel in step 215. The output of step 215 can be used as input of the second torque balance 204.

The calculated angular rate and the calculated angle can be used to determine a friction torquer^ in step 216. The friction torque may also be used as input of the second torque balance 204.

Thus, the second torque balance may perform the following operation:

A third torque balance may then be determined to obtain the feedback torque to be applied T_r¾ in step 217.

This third torque balance performs the following operation:

The feedback torque to be applied can then be applied to the steering wheel.

The above method can be implemented in a simulator such as a driving simulator equipped with a steering wheel on which the feedback torque T_r¾ can be applied using an actuator such as a motor. The model simulates the rack force values and other values of components of a vehicle which is simulated and of which only a physical steering wheel may exist. While the solutions of the prior art may suffer from oscillations, feedback ripples, and wheel shimmy, the above method which measures an actual torque reduces these issues.

The above method has been tested on drivers in driving simulators and has shown satisfactory results. A comparison between a solution according to the prior art, for example one in which a torque to be applied is determined solely according to a steering wheel angle and an angular rate as sole measured inputs to calculate a feedback torque request, has been carried out on a test group of 8 drivers. This test group had an average age of 32 years (standard deviation=7.36). On average participants had the driving license for 13 years (standard deviation=7.56). Statistical significance of subjective indicators was assessed through analysis of variance (ANOVA). Participants declared a reduction in overall effort of 34% and increase in steering realism of 29% with torque control compared to the traditional control. They reported a reduction in steering wheel oscillations of 43% and an increase in steering wheel holding precision of 30% with the new control compared to the method according to the prior art. The method of figure 2 may also be applied in a vehicle using a steer by wire module to provide drivers a steering feedback amount with more fidelity. In such an alternative, the rack force can be measured by means of a rack axial force sensor or using estimation techniques (if the model requires these values). The steer by wire logic will then aid the driver's input torque to counter the rack forces coming from the tires, using the value T_r¾.

Figure 3 is an alternative embodiment of a method 300 which can be used in a vehicle equipped with a power assist module.

In this method, the power assist module delivers a torque T_assist to be applied and this value can be multiplied by the gear ratio /^between an electric power steering motor and the steering column (step 301).

Then, a torque balance 302 having an operation similar to the second torque balance of figure 2 is determined using T_assist multiplied by i_g, as well as: p_t¾^c, the calculated torque at the motor gear for the power steering system,

T%^eas, the torque measured at the torsion bar, the torsion bar being the most flexible element of the steering column, and whose deformation is used to measure the torque in many steering systems, , C_sw5s_w ^rt _/ as defined in reference to figure 2, T , as defined in reference to figure 2,

Tcom_p ' as defined in reference to figure 2.

The output of the torque balance of step 302 is /^^rt ¾^rt. The determination of T_comp, d^, and

is performed similarly as in figure 2 in steps 303, 304, 305, 306, 307, 308, 309, and 310 which are respectively similar to steps 205 to 212.

A final torque balance is determined (step 314) using T_comp and Tassis_t to obtain T™¹ the actual torque which will be applied to the power steering motor and which is a feedback torque. The method of figure 3 has been shown to reduce steering wheel shimmy and torque fluctuations at the steering wheel. In the above example, the difference between calculated and measured column angles and rate is proportional to the disturbance affecting driver's feel. hFigure 4 is a graphical representation of the evolution of the steering wheel angle against the steering wheel torque for a method according to the prior art (for example one in which a torque to be applied is determined solely according to a steering wheel angle and an angular rate as sole measured inputs to calculate a feedback torque request, designated as "traditional position based control" on the figure) and for the method of figure 2 (designated as new torque based control on the figure).

As can be seen on the figure, the method provides a reduction in amount and amplitude of the steering torque feedback ripples.

Figure 5 is the power spectral density of torsional vibrations measured using accelerometers mounted on the control loading system shaft. A reduction of noise and system resonances due to steering wheel oscillations can be observed between the method of the prior art and the method of figure 2 (same references as in figure 2). Figure 6 is a schematic representation of a system such as a simulator or a vehicle 400 comprising a device 401 configured for example to perform the method of figures 2 or 3.

The device 401 comprises a model 402, for example stored in a nonvolatile memory in the form of computer instructions executable on a processor of the device.

This device further comprises: a module 403 for acquiring measured values of an angle and of an angular rate of the steering wheel, and of a torque associated with the steering wheel, a module 404 for acquiring, using a virtual model of the steering system and of the vehicle, calculated values of an angle and of an angular rate of the steering wheel, and of at least one calculated torque associated with the steering system, a module 405 for determining said feedback torque to be applied using the measured values and the calculated values.

These modules 403 to 405 may also be stored in a nonvolatile memory in the form of computer instructions executable on a processor of the device. Consequently, the modules 403 to 405 may form a computer program.

The output of module 405 can be applied to an actuator 407 which applies a torque on a steering wheel 406.

Although the present invention has been described above with reference to certain specific embodiments, it will be understood that the invention is not limited by the particularities of the specific embodiments. Numerous variations, modifications and developments may be made in the above-described embodiments within the scope of the appended claims.

Claims

1. A method for determining a feedback torque to be applied to a steering wheel configured to be used in a steering system of a vehicle, comprising: acquiring measured values of an angle (5™^eas) and of an angular rate of the steering wheel, and of a torque(r_s^^eas) associated with the steering wheel, acquiring, using a virtual model of the steering system and of the vehicle, calculated values of an angle (<¾ ) and of an angular rate (5^^rt)of the steering wheel, and of at least one calculated torque (r ^) value associated with the steering system, determining (217, 314) said feedback torque to be applied using the measured values and the calculated values.

2. The method of claim 1, further comprising determining a first torque balance using at least the measured torque associated with the steering wheel and a compensation torque (T_comp) compensating a disturbance, the first torque balance delivering a value of the torque applied by the driver

3. The method of claim 2, wherein the compensation torque compensating a disturbance is obtained by determining the difference between the calculated and the measured angle values and the difference between the calculated and the measured angular rate values.

4. The method of claim 2 or 3, wherein determining said feedback torque to be applied to the steering wheel is performed using at least the compensation torque compensating a disturbance, and the at least one calculated value of the torque associated with the steering wheel.

5. The method of any one of claims 1 to 4, further comprising determining a second torque balance using at least the value of the torque applied by the driver and/or the calculated value of the at least one torque associated with the steering wheel, so as to obtain at least a calculated angular acceleration value of the steering wheel (¾^rt).

6. The method of claim 5 in combination with claim 3, wherein the calculated angular rate and the calculated angle are respectively obtained by integrating the calculated angular acceleration and integrating the calculated angular rate.

7. The method of any one of claims 2 to 6, wherein determining the feedback torque to be applied to the steering wheel comprises determining a third torque balance using the values obtained from the calculated angular acceleration value of the steering wheel and/or the compensation torque.

8. The method of any one of claims 1 to 7, wherein determining the feedback torque to be applied to the steering wheel comprises taking into account a friction parameter of the steering wheel and/or a damping parameter (C_sw) of the steering wheel.

9. The method of any one of claims 1 to 8, wherein the steering wheel is a steering wheel of a simulator.

10. The method of any one of claims 1 to 8, wherein the steering wheel is a steering wheel of a vehicle equipped with a steer by wire module.

11. The method of any one of claims 1 to 8, wherein the steering is a steering wheel of a vehicle equipped with a power steering system of a vehicle.

12. A device for determining a feedback torque to be applied to a steering wheel configured to be used in a steering system of a vehicle, comprising: a module (403) for acquiring measured values of an angle and of an angular rate of the steering wheel, and of a torque associated with the steering wheel, a module (404) for acquiring, using a virtual model of the steering system and of the vehicle, calculated values of an angle and of an angular rate of the steering wheel, and of at least one calculated torque associated with the steering system, a module (405) for determining said feedback torque to be applied using the measured values and the calculated values.

13. A system comprising the device of claim 12.

14. A computer program including instructions for executing the steps of a method according to any one of claims 1 to 11 when said program is executed by a computer.

15. A recording medium readable by a computer and having recorded thereon a computer program including instructions for executing the steps of a method according to any one of claims 1 to 11.